Rhytisma

Rhytisma is a genus of fungi belonging to the order Rhytismatales. These fungi are plant pathogens, primarily affecting woody plants. The most well-known species is Rhytisma acerinum, which causes tar spot disease on maple trees. This disease is characterized by the formation of black, tar-like spots on the leaves. These spots are actually stromata, which are hardened masses of fungal tissue containing the fruiting bodies.

The life cycle of Rhytisma typically involves overwintering in fallen leaves as a mycelial mass or as the stromata themselves. In the spring, the stromata mature and produce spores. These spores are then released and dispersed by wind and rain, infecting new leaves. While tar spot is visually striking, it is generally considered a cosmetic disease and rarely causes significant harm to healthy, mature trees. Younger or stressed trees may be more susceptible to damage.

Other species of Rhytisma affect a range of hosts, including conifers and other deciduous trees. Their symptoms can vary but often involve leaf spots or lesions. Research into the genetics and ecological roles of Rhytisma continues to expand our understanding of these widespread fungal pathogens. Control measures for tar spot are usually unnecessary, but in severe cases, good sanitation practices, such as removing fallen leaves, can help reduce the inoculum.